Peripheral nerve blocks are performed by injecting local anesthetic solution very near, but not inside, major nerves or plexuses. When properly performed, these blocks can provide surgical anesthesia and/or postoperative pain relief. General anesthesia and/or narcotics can often be avoided or minimized, thus reducing potential side effects. Unfortunately, if the local anesthetic is mistakenly injected inside the nerve, rather than next to it, the result can be permanent nerve damage.
Various techniques and technologies have evolved over the past century in an effort to improve the success and safety of regional anesthesia. Blind techniques, relying on paresthesias or “clicks and pops” being sensed by the practitioner as the needle is inserted, were supplanted by the use of peripheral nerve stimulation to elicit a motor response when the block needle approached the nerve. In turn, nerve stimulation is now being replaced in many treatment centers by direct visual guidance using high-frequency ultrasound. While ultrasound guidance has greatly improved the speed and reliability of peripheral blocks, the possibility of intraneuronal injection with permanent catastrophic neuropathy still remains a concern. For example, if the block needle is angled slightly relative to the ultrasound beam, its tip may fall outside the beam, and not be visible as it enters a nerve. Dr. Admir Hadzic and colleagues have shown that it is actually injection pressure that determines whether or not a nerve will be damaged by intraneuronal injection (Reg. Anesth. Pain Med. 2004; 29:417-423). Specifically, it was determined that it is unsafe to exceed 20 psi during the injection. Injection pressures below that threshold failed to damage the nerves, even when the needle had penetrated the wall, and was definitely inside the nerve at the time of injection.
U.S. Pat. No. 6,866,648, entitled METHOD AND APPARATUS TO DECREASE THE RISK OF INTRANEURONAL INJECTION DURING ADMINISTRATION OF NERVE BLOCK ANESTHESIA, by Dr. Admir Hadzic, et al., the teachings of which are expressly incorporated herein by reference, describes a system for more safely administering injections using a syringe that incorporates a “pop-up” valve assembly in line with the injection tubing and associated fluid path. The valve pops up to provide a visual cue that a predetermined safe injection pressure has been exceeded. A version of this device is marketed under the name BSMART™ by Concert Medical LLC, Norwell, Mass. While this device is useful—particularly in raising awareness as to the need for a safety-injection system, it possesses at least two significant shortcomings. First, the pop-up valve must be continually observed by the practitioner while the injection is being made. This is not practical in a real-world clinical setting. Rather, distractions are possible in any setting, and with the use of ultrasound guidance all eyes tend to be on the ultrasound screen—not the syringe. Second, while the Hadzic device may indicate excessive pressure during the injection, it does not prevent it. The danger of permanent nerve damage, while reduced, still remains completely in the hands of the practitioner.
Another earlier attempt to provide a safety syringe is disclosed in U.S. Pat. No. 4,759,750, entitled PRESSURE SENSING SYRINGE, by James H. DeVries, et al., the teachings of which are expressly incorporated herein by reference. This device provides a pop-up projection within the center of the syringe plunger thumb plate. This projection is biased through the thumb plate, and painlessly into the practitioner's thumb, if the injection pressure fed-back though the syringe barrel exceeds a predetermined level. Thus, this device avoids the disadvantageous requirement that the practitioner split his or her attention between the syringe and the ultrasound display as encountered in Hadzic by providing a tactile, rather than a visual, indicator of excess injection pressure. However, this device also fails to prevent an over-pressurized injection. In addition, different practitioners' sensitivity and response time to an over-pressure indication may vary widely. Some practitioners may simply not feel the indicator, or only feel it after significant over-pressure has occurred—particularly through thickened surgical gloves.
It is, therefore, desirable to provide a safety syringe, applicable to the administration of neural blocks and other forms of hypodermic injection, which provides a tactile indication of an injection pressure that exceeds a predetermined level. It is further desirable that the safety syringe allow the practitioner to maintain visual attention of the injection site and/or an imaging display of the site, such as employed in ultrasound device. Moreover, it is desirable to provide a safety syringe system, and method for using such a system, which actively prevents the administration of an injection that exceeds a predetermined fluid pressure. The system should be adapted so that the fluid pressure limit is variable for differing types of injections/syringes, and should be easily manufactured, facilitating its use as a generally disposable item.